Many modern electronic devices, especially low-power portable devices, demand small antennas to transmit and receive wireless signals. As antennas decrease in size, however, their bandwidth narrows, and it becomes increasingly important to ensure that they are properly tuned to a desired frequency (i.e., the antenna's resonant frequency is adjusted to match the frequency of an incoming signal). But, even an antenna that is perfectly tuned initially is subject to environmental factors that may adversely affect its frequency and/or impedance, and, furthermore, an application may require an antenna to transmit and receive signals over multiple frequencies. Therefore, an automatic tuning system may be necessary to dynamically tune the antenna to a desired frequency and/or adjust the impedance of the antenna. One element typically present in an automatic tuning system is a means for determining the resonant frequency and impedance match of the antenna, preferably without introducing appreciable loss into the system.
One approach for automatic tuning is to inject a signal into an antenna input connection and observe the reflected signal. This approach generally requires introducing a network and/or a circuit element (e.g., a coupler or a switch) between the antenna and the system using the antenna (e.g., a radio). Any approach, however, that introduces circuitry between the antenna and the system that uses the antenna will generally introduce loss into the system, which degrades the system noise figure and dissipates transmit power. The loss in transmit power may be compensated for, but, for systems where power is a limited resource, such as battery-operated systems, the penalty may be a significant increase in battery size or a large drop in battery life. Unlike the loss in transmit power, the impact on the system noise figure when a signal is received at the antenna is typically irreversible, and cannot be compensated for.
Other automatic tuning techniques involve monitoring the phase difference between the current and voltage at the antenna, which will approach zero when the antenna is at resonance, or using a separate, inductively coupled loop to sense the tuning of a loop antenna. Observing just the signals at the input of an antenna, however, whether through the current/voltage phase relationship or through the forward and reflected signals, does not allow determination of the quality of circular polarization. Many circularly polarized antennas have two modes in quadrature, and the amplitude and phase relationship cannot be easily determined from impedance measurements at the antenna input(s) alone.
A need therefore exists for a low-power, low-loss antenna tuning detection system that is capable of determining the tuning state of modern antennas, including ones with multiple modes.